Burnham Alliance wiring/controls

Burnham Mega steam with Alliance and two hot water heating zones

Just installed a Burnham Mega Steam 396 boiler using the tankless coil for two small hot water heating zones. I used Honeywell RA832A relays for these two zones. The boiler and the two zones work great! Now I want to install the Alliance Indirect water heater. The aquastat for the heating zones is mounted into the tankless coil tapping and is wired into the burner controls. I purchased a third RA832A relay but don't know how to wire it into the TPI control on the Alliance water heater. I want to make the water heater the priority zone. The aquastadt for the Alliance is installed into the L tapping on the back of the boiler. How does it get wired into the relay and how do I wire this relay so that it is a priority over the other two zones?